IB Chemistry on Nuclear Magnetic Resonance (NMR) Spectroscopy
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IB Chemistry on Nuclear Magnetic Resonance (NMR) Spectroscopy

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IB Chemistry on Nuclear Magnetic Resonance (NMR) Spectroscopy and Chemical shift

IB Chemistry on Nuclear Magnetic Resonance (NMR) Spectroscopy and Chemical shift

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  • 1. Electromagnetic Radiation and Spectroscopy Electromagnetic Radiation Interact with Matter (Atoms, Molecules) = Spectroscopy Electromagnetic Radiation Radiowaves Infra Red UV or visible Nuclear spin Molecular vibration Transition of outer most valence electrons Nuclear Magnetic Resonance Infra Red Spectroscopy UV Spectroscopy Atomic Absorption Spectroscopy Spectroscopy• Organic structure determination • Organic structure determination • Quantification of metal ions• MRI and body scanning • Functional gp determination • Detection of metal in various samples • Measuring bond strength • Measuring degree unsaturation in fat • Measuring level of alcohol in breath
  • 2. Nuclear Magnetic Resonance Spectroscopy (NMR)Nuclear Magnetic Resonance Spectroscopy (NMR)• Involve nucleus (proton + neutron) NOT electrons• Proton + neutrons = Nucleons• Nucleons like electrons have spin and magnetic moment (acts like a tiny magnet) Nuclei with even number of nucleon (12C and 16O)• Even number of proton and neutron – NO net spin• Nucleon spin cancel out each other –Nucleus have NO overall magnetic moment – NOT absorb radiowave radiation Spin cancel each other Nuclei with odd number of nucleon (1H, 13C, 19F, 31P)-Nucleon have net spin – Nucleus have NET magnetic moment – Absorb radiowave radiation• Nuclei with net spin – magnetic moment will interact with electromagnetic radiation/radio waves• Nuclei have a “spin” associated with them (i.e., they act as if they were spinning about an axis) due to the spin associated with their protons and neutrons.• Nuclei are positively charged, their spin induces a magnetic field• NMR spectroscopy does not work for nuclei with even number of protons and neutrons— nuclei have no net spin.
  • 3. Nuclear Magnetic Resonance Spectroscopy (NMR) Main features of HNMR Spectra 1. Number of different absorption peaks – Number of different proton/chemical environment 2. Area under the peaks - Number of hydrogen in a particular proton/chemical environment (Integration trace) - Ratio of number of hydrogen in each environment 3. Chemical shift - Chemical environment where the proton is in - Spinning electrons create own magnetic field, creating a shielding effect - Proton which are shielded appear upfield. (Lower frequency for resonance to occur) - Proton which are deshielded appear downfield away. (Higher frequency for resonance to occur) - Measured in ppm (δ) 4. Splitting pattern - Due to spin-spin coupling - The number of peak split is equal to number of hydrogen on neighbouring carbon +1 (n+1) peak NMR spectrum of CH3CH2Br Chemical Shift Number of peaks Splitting pattern Area under peaks Chemical shifthttp://chemwiki.ucdavis.edu/Physical_Chemistry/Quantum_Mechanics/Atomic_Theory/Electrons_in_Atoms/Electron_Spin
  • 4. Nuclear Magnetic Resonance Spectroscopy (NMR) Absence of External magnetic Field (EMF) Presence of External Magnetic Field (EMF) • The TWO nuclear spin have the same energy, (same energy level) • The TWO nuclear spin split to TWO different energy levelPresence of External Magnetic Field (EMF) • External magnetic field applied to atomic nuclei, magnetic field of nuclei align themselves either with or against magnetic field • Nuclei have a slight preference for the parallel alignment with the applied field as it has a slightly lower energy, • Nuclei can absorb energy to move/flip to higher energy level by absorbing energy in radio frequency region High spin nuclei align against magnetic field Presence of EMF ∆E • Two spins in different energy level • Lower spin nuclei absorb radio frequency equivalent to ∆E • Move to higher energy level Absence of EMF • Two spins in same energy level Lower spin nuclei align with magnetic field
  • 5. Chemical Shift (Shielding Effect) Proton in nucleus – have spin – generate its magnetic field (MF) Electrons around nucleus – have spin- also generate its magnetic field Protons shielded by MF produced by electrons will appear UPFIELD Protons deshielded by electron withdrawing gps will appear DOWNFIELDDownfield Upfield Presence of EMF • Two spins in different energy level • Lower spin nuclei absorb radio frequency equivalent to ∆E • Move to higher energy level Without any SHIELDING EFFECT ∆E • Energy of ∆E absorb by H to move to higher energy level Absence of EMF Presence of EMF • Two spins in same energy level • Two spins at diff energy level SHIELDING EFFECT • Electrons around H will produce MF and shield the H • H in CH3 will experience less EMF (SHIELDED) ∆E is smaller • Absorb at lower radiofrequency to move to higher level • ∆E absorb by H to move to higher energy level is less • Appear upfield. Absence of EMF Presence of EMF • Two spins in same energy level • Two spins at diff energy level
  • 6. Chemical Shift (Deshielding Effect) Proton in nucleus – have spin – generate its magnetic field (MF) Electrons around nucleus – have spin- also generate its magnetic field Protons shielded by MF produced by electrons will appear UPFIELD Protons deshielded by electron withdrawing gps will appear DOWNFIELDDownfield Upfield Presence of EMF • Two spins in different energy level • Lower spin nuclei absorb radio frequency equivalent to ∆E • Move to higher energy level ∆E Without any DESHIELDING EFFECT • Energy of ∆E absorb by H to move to higher energy level Absence of EMF Presence of EMF • Two spins in same energy level • Two spins at diff energy level DESHIELDING EFFECT • Electrons are withdrawn away by C=O gp • Carbonyl gp has electron withdrawing effect • Less electron around the H in CH3 ∆E is higher • H in CH3 will be deshielded, experience greater EMF • ∆E absorb by H, to move to high energy level is higher • Absorb at higher radiofreq, to move to high level • Appear downfield Absence of EMF Presence of EMF • Two spins in same energy level • Two spins at diff energy level
  • 7. Chemical Shift (Shielding and Deshielding Effect) Absence of EMF Presence of EMF • Two spins in same energy level • Two spins at diff energy levelDeshielding Effect DESHIELDING EFFECT • Electrons are withdrawn away by C=O gp ∆E is higher • Carbonyl gp has electron withdrawing effect • Less electron around the H in CH3 • H in CH3 will be deshielded, experience greater EMF • ∆E absorb by H, to move to high energy level is higher • Absorb at higher radiofreq, to move to high level • Appear downfieldNo shielding ∆E Without any SHIELDING EFFECT • Energy of ∆E absorb by H to move to higher energy levelShielding Effect SHIELDING EFFECT • Electrons around H will produce MF and shield the H • H in CH3 will experience less EMF (SHIELDED) • Absorb at lower radiofrequency to move to higher level ∆E is smaller • ∆E absorb by H to move to higher energy level is less • Appear upfield. Downfield Upfield
  • 8. Chemical Shift (Shielding and Deshielding Effect) Shielding/Deshielding: • Electron circulates nucleus, creates magnetic field opposing the external magnetic field. • Hence, each nucleus experience a slightly different magnetic field • (Sum of external field and field from the electron cloud). • Energy a nucleus achieves resonance depends on its surroundings. • Frequency absorption depend on electron density around nucleus (chemical environment) Chemical shift of various electron withdrawing groupsDownfield Upfield • Electron withdrawn from CH3 by C=O • Deshield the H in CH3 • Absorb at slightly higher radiofreq • Upfield ≈ 2.1 • Electron withdrawn from CH2 by COO • Stronger electron withdrawing effect • Higher ↑ Deshielding effect on H in CH2 • Absorb at Higher ↑ radiofreq • Slightly Downfield ≈ 4.1 • Electron withdrawn by benzene • Stronger electron withdrawing effect • Higher ↑ deshielding effect on H • Absorb at Very high ↑ radiofreq • Very Downfield ≈ 7.3 - 8 • Electron withdrawn by COOH • Electron withdrawn from H by CHO • Very strong electron withdrawing effect • Very strong electron withdrawing effect • Highest deshielding effect on H • Higher ↑ Deshielding effect on H in CHO • Absorb at Very High↑ radiofreq • Absorb at Very High↑ radiofreq • Very Very Very Downfield ≈ 12 • Very Very Downfield ≈ 9.7
  • 9. Nuclear Magnetic Resonance Spectroscopy (NMR) HO-CH2-CH3 OH CH2 CH3 • chemical shift ≈ 4.8 • chemical shift ≈ 3.8 • chemical shift ≈ 1 • integration = 1 H • integration = 2 H • integration = 3 H • No split (Singlet) • split into 4 • split into 3 1 2 3 12 Upfield Downfield • 3 different proton environment • Ratio of 3:2:1 Tetramethyl Silane (TMS) as STD •Strong peak upfield (shielded) •Silicon has lower EN value < carbon • Electron shift to carbon • H in CH3 will be more shielded • Experience lower EMF, absorb ↓ radiofrequancy • UPFIELD ≈ 0 Advantages using TMS • Volatile and can be removed from sample • All 12 hydrogens are in the same proton environment • Single strong peak, upfield, doesnt interfere with other peaks • All chemical shift, measured in ppm (δ) are relative to this STD, taken as zeroClick here for more complicated proton chemical shift
  • 10. NMR Spectrum O ║ CH3-C-O-CH2-CH3 B A C 2 3 3 3 diff proton enviroment, Ratio H - 3:2:3 • Peak A – split to 3 – 2H on neighbour C • Peak B - No split • Peak C – split to 4 – 3H on neighbour C O ║ HO-C-CH2-CH3 A BC 1 2 312 3 diff proton enviroment, ratio H - 3:2:1 • Peak A – split to 3 – 2H on neighbour C • Peak B – split to 4 – 3H on neighbour C • Peak C – No split
  • 11. NMR Spectrum HO-CH2-CH3 A BC 3 2 1 3 diff proton enviroment, Ratio H - 3:2:1 • Peak A – split to 3 – 2H on neighbour C • Peak B – split to 4 – 3H on neighbour C • Peak C – No split O ║ CH3-C-CH2-CH2-CH3 A D C B 2 3 2 3 4 diff proton enviroment, Ratio H - 3:2:2:3 • Peak A – split to 3 – 2H on neighbour C • Peak B – split to 6 – 5H on neighbour C • Peak C – No split • Peak D – split to 3 – 2H on neighbour C
  • 12. NMR Spectrum O ║ CH3-C-O-CH2-CH2-CH3 A D C B 2 3 2 3 4 diff proton enviroment, Ratio H – 3:2:2:3 • Peak A – split to 3 – 2H on neighbour C • Peak B – split to 6 – 5H on neighbour C • Peak C – No split • Peak D – split to 3 – 2H on neighbour C O ║ H-C-CH3 AB 3 19.8 2 diff proton enviroment, Ratio H - 3:1 • Peak A – split to 2 – 1H on neighbour C • Peak B – split to 4 – 3H on neighbour C
  • 13. NMR Spectrum CH3Molecule with plane of symmetry | H-C-OH | CH3 C A B 1 1 6 3 diff proton enviroment, Ratio H - 6:1:1 • Peak A – split to 2 – 1H on neighbour C • Peak B – No split • Peak C – split to 7 – 6H on neighbour C O CH3Molecule with plane of symmetry ║ | CH3-C-O-CH | CH3 A B C 3 6 1 3 diff proton enviroment, Ratio H - 6:3:1 • Peak A – split to 2 – 1H on neighbour C • Peak B – No split • Peak C – split to 7 – 6H on neighbour C
  • 14. NMR Spectrum OMolecule with plane of symmetry ║ CH3-CH2-C-CH2-CH3 A B 4 6 2 diff proton enviroment, Ratio H – 6:4 • Peak A – split to 3 – 2H on neighbour C • Peak B – split to 4 – 3H on neighbour C O CH3Molecule with plane of symmetry ║ | H-C-C-CH3 | A CH3 B 9 1 2 diff proton enviroment, Ratio H – 9:1 • Peak A – No split • Peak B – No split
  • 15. NMR Spectrum CH3Molecule with plane of symmetry | HO-CH2-CH | A CH3 B D C 2 1 1 6 4 diff proton enviroment, Ratio H – 6:1:1:2 • Peak A – split to 2 – 1H on neighbour C • Peak B – split to 7 – 6H on neighbour C • Peak C – No split • Peak D – split to 2 – 1H on neighbour C CH3-CH-CH3Molecule with plane of symmetry | CI A B 1 6 2 diff proton enviroment, Ratio H – 6:1 • Peak A – split to 2 – 1H on neighbour C • Peak B – split to 7 – 6H on neighbour C
  • 16. IR Spectra search for different Organic MoleculesClick here to animated Spectra Click here to search IR spectra